structural adhesives such as cyanoacrylate epoxy resins and the like are well-known for providing strong and permanent bonds. However, before curing, such adhesives do not normally provide aggressive adhesive qualities and, therefore, require external aids such as clamping devices to hold the substrates to be bonded together until cure has been completed and a structural bond formed.
Tacky, pressure-sensitive adhesives have been known for many years and have been used in various bonding and fastening applications. They provide a flexible bond and are used in a wide range of applications including adhering labels, decals, and bonding automotive trim parts and name plates to various substrates. However, they show substantially lower strength characteristics compared to a structural adhesive like cured epoxy or cyanoacrylates.
There is a need in the industry to find an adhesive composition which exhibits pressure-sensitive adhesive characteristics useful during assembly and which may then be converted by subsequent treatment to a structural adhesive. It is preferred that the subsequent treatment be by thermal means and could even be cured by electromagnetic induction if desired.
In use, such adhesive could be provided between two release layers or as a single tape with differential release, preferably using a release layer which is differentially releasable from an adhesive layer bonded to a backing which may also be a release surface. The adhesive could then be conveniently applied between the surfaces to be bonded. Sufficiently firm contact or light pressure established between the surfaces will cause sufficient adhesion to temporarily hold the assembly. Thereafter, heat is applied to the composite structure to convert the intermediate adhesive layer into a structural adhesive bond.
Applications for such tapes include "hem-flange" bonding and weld reinforcement in automotive, reinforcing materials for strengthening thin sheet metal and plastics by forming integral structures with the original substrate, and bonding of plastics, precoated metals, SMC, and other advanced materials.
Several attempts were made in the past to come up with a pressure-sensitive adhesive which changes into a structural adhesive after heat activation.
U.S. Pat. No. 3,326,741 (1967) to Olson discloses a tacky pressure-sensitive adhesive which on heat activation achieves a strong permanent bond. The patent discloses a nitrile rubber/epoxy resin blend with curing agent such as dicyandiamide. However, the films did not show good cohesive strength at room temperature which is typically required for a pressure-sensitive adhesive.
U.S. Pat. No. 3,639,500 (1972) to Muny and assigned to Avery Dennison Corporation discloses a curable pressure-sensitive adhesive composition containing a polyepoxide, a carboxylated diene polymer, and an acrylic ester tackifier which on heat activation provides a structural bond.
U.S. Pat. No. 4,404,246 (1983) to Charbonneau et. al discloses an alkoxylated amino formaldehyde condensate in an acrylic pressure-sensitive adhesive composition as a latent crosslinking agent to improve the cohesive strength after heat activation. However, the material behaves as a highly crosslinked pressure-sensitive adhesive after heat activation and cannot be used for structural bonding.
U.S. Pat. No. 4,452,955 to Boeder discloses an adhesive composition consisting of a polymer dissolved in polymerizable monomer, an accelerator such as organic sulfimides and perfluoroalkylsulfonanilides, and an inhibitor. The adhesive shows pressure-sensitive adhesive properties, and after heat activation, the adhesive demonstrates properties similar to a structural adhesive. However, the properties as detailed in the examples do not show properties of true structural type adhesives, especially in lap shears.
U.S. Pat. No. 4,404,345 (1983) to Janssen also discloses a similar adhesive composition consisting of an adhesive base as the first part and an initiator portion as the second part. Bonding methods are also described.
None of the prior art described above teaches or suggests a pressure-sensitive adhesive that can be heat cured to form the strong and permanent bond of a structural adhesive.